Horses experiencing exercise-induced pulmonary hemorrhage (EIPH), also known as “bleeding,” represent a serious problem to the horse racing industry. Bleeding is a condition in which the tiny blood vessels in a horse's lungs rupture due to stress sustained during intense physical exertion. Studies of horses in training and those in competition at racetracks have shown that from about 70% to 100% of them experience EIPH after performing. This has been shown both endoscopically (Pascoe et al. 1981; Sweeney, 1991) and from trans-tracheal washings (Whitwell and Greet, 1984). EIPH occurs in three variations. First, there is simple EIPH which is an acute condition resulting from the strain of exercise. Second, there is patent pulmonary hemorrhage (hereinafter “PPH”), which involves bleeding in the lungs as a reaction to an allergen, infection, or due to hypertension. Third, equine specimens experience composite bleeding, which is the result of the combined effects of simple EIPH and PPH.
Although numerous hypotheses have been proffered, it is generally accepted by the scientific community that simple EIPH occurs as the natural consequence of strenuous running due to the layout of the horse's organs and the way the equine body moves during high-speed galloping. The front half of a horse contains the heart, lungs and other major organs while the posterior half contains the intestines, which are suspended within the abdomen by ligaments. The rationale is based on studies by M. Manohar (Am J. Vet. Res, 1993, 54:142-146) and West et al. (J. Appl. Physiol., 1991, 71:573-582 and J. Appl. Physiol 1993, 75: 1097-1109) among others who have demonstrated that excessive pulmonary artery pressure and stress failure at the pulmonary capillary level is due to increased transmural pressure during strenuous exercise of the equine. Due to the back-and-forth motion of galloping, the horse's intestines swing like a pendulum at the end of the ligaments. When running at full speed, especially in the fastest sprint races, the movement of the intestines can get out of phase with the movement of the diaphragm in such a way that the intestinal mass is swinging forward as the horse is trying to exhale. This causes the diaphragm to be slammed forward and slightly upward. The diaphragm, in turn, squeezes part of the lung against the chest wall.
The lungs are filled with alveoli, tiny air sacs, and capillaries, miniscule blood vessels. The alveoli and capillaries are so fine and so interconnected that oxygen from the inhaled air can pass into the bloodstream, and carbon dioxide in the blood can pass out of the blood into the lungs to be exhaled. The capillaries are at their smallest and most efficient near the rear, tapered end of the lungs where they are in close proximity to the diaphragm. These extremely fine capillaries are repeatedly impacted by the forward-surging intestinal mass. As they rupture under the stress, the horse's air passages become clogged with blood. This causes difficulty in breathing which causes reduced athletic performance and/or a shortened athletic career. Thus, EIPH is one of the most serious veterinary problems facing the horse racing industry.
EIPH is defined as bleeding from the lungs as a consequence of exertion. Most horses involved in competitive racing experience EIPH. The incidence of EIPH ranges from about 30% for standardbreds and polo ponies to greater than 60% for Thoroughbreds, Quarterhorses, Appaloosas and Arabians. The minimum level of exertion needed to induce EIPH is unknown. Though it has been observed in some Thoroughbred horses after trotting, cantering and slow training gallops, EIPH is generally associated with more strenuous exertion, such as competitive flat racing, pacing, trotting, jumping or barrel racing. Moreover, EIPH is most prevalent in Thoroughbreds.